This invention relates to an engine induction system provided with a mechanical supercharger disposed in an intake manifold and a swirl control valve disposed in one of two inlet ports provided for each cylinder.
There has been known an induction system having the following construction as, for example, disclosed in Unexamined Japanese Patent Publication No. 1-104920. In this system, a mechanical supercharger for supercharging intake air to be admitted into combustion chambers defined in respective cylinders is disposed in an upstream portion of an intake manifold. In addition, two inlet ports are formed at a downstream portion of the intake manifold so as to be communicated with a corresponding cylinder, and inlet valves are provided at downstream end openings of the respective inlet ports. In one of the two inlet ports for each cylinder is disposed a swirl control valve which is opened at least in a high speed/high load region. In an example shown in FIG. 4 of the above publication, the swirl control valve is opened in a high speed range and a low speed/high load region while closed in low and medium load regions of a low speed range.
Further, in the above system, one of the inlet valves for each cylinder is opened at an earlier timing than the other, and thereby an overlap period of openings of the inlet and exhaust valves for the cylinder is set long. Hereinafter, such an overlap period will be referred to as a valve timing overlap period. The swirl control valve is disposed in the inlet port corresponding to the inlet valve opened at a delayed timing. While the swirl control valve is opened, exhaust gas in a combustion chamber is sufficiently scavenged during a long overlap period by introducing the supercharged intake air into the combustion chamber from the supercharger.
However, this induction system suffers the following problems. When the swirl control valve is opened in a low speed/high load region where the pressure of the supercharged intake air (hereinafter referred to as supercharged pressure) is high, the intake air is liable to the blown through the combustion chamber without being used for combustion though improved scavenging may be obtainable. Also, in a low speed/medium load region where the supercharged pressure is lower than in the low speed/high load region and a considerable output performance is required, closure of the swirl control valve results in insufficient scavenging. This is disadvantageous in terms of the output performance and suppression of knocking.
Further, if the swirl control valve is disposed in the inlet port corresponding to the inlet valve opened at the earlier timing, the valve opening overlap period is shortened when the swirl control valve is closed. Accordingly, it becomes difficult to attain improved scavenging.
Unexamined Japanese Patent Publication No. 61-218726 discloses an induction system having two inlet ports in communication with a corresponding cylinder, a swirl control valve is disposed in one of the two inlet ports for the cylinder, and respective inlet ports are opened at different timings. However, this publication does not sufficiently mention the scavenging efficiency in the case where the above arrangement is combined with a supercharger. Further, Unexamined Japanese Patent Publication No. 61-58920 discloses an induction system having two inlet valves and two exhaust valves in communication with a corresponding cylinder, the inlet valves are differently sized and opened at different timings, and the exhaust valves are differently sized and opened at different timings. However, this publication does not teach a concept of a combined use of a supercharger and a swirl control valve. Accordingly, these induction systems have more room for improvement in supercharging according to an operating condition and scavenging efficiency.